The present invention relates to a method for the preparation of a spongy metallic body or, more particularly, to a method including the step of pressurized casting for the preparation of a spongy metallic body having a high porosity with intercommunicating open cells and useful as a functional material in various fields of applications.
As is known, porous metallic bodies in general are classified into those of closed-cell structure, in which each of the void cells is isolated from the others with the surrounding metal walls not to permit permeation or penetration of gases and liquids through the body, and those of open-cell structure, in which each of the cells intercommunicates with the others to permit permeation or penetration of gases and liquids through the body like a sponge and the porous metallic bodies of each class are employed in the fields in which their respective characteristic properties are utilizable. Spongy metallic bodies of open-cell structure are useful as a high-functional material in various fields of applications such as oilless bearings, filter members, heat-exchange elements, electrodes, catalysts, reservoirs of specific substances and the like as only a few of the examples in which absorption or penetration of gases or liquids is essential.
The most conventional method for the preparation of a spongy metallic body is the so-called powder metallurgical process in which a metal powder is shaped by molding and the molded green body is subjected to sintering. One of the problems in such a sintering method is the limited porosity of the sintered metallic body. Since the packing density or space impregnation of the metal particles in the molded green body is the predominant determinant of the porosity in the finished sintered body, namely, the porosity of the spongy metallic body obtained by this method can rarely exceed about 40%. In addition, the open-cell structure of such a spongy metallic body is subject to deterioration by a mechanical working of the body such as cutting, grinding and plastic deformation to always cause clogging or collapsing of the open cells resulting in the decreased permeability of gases or liquids. This problem necessarily presents a limitation in the shapes of the products that the shape of the final product in most cases is a mere reproduction of the shape of the molded green body so that sintered products of complicated forms can hardly be manufactured by this method.
An alternative method for the preparation of a spongy metallic body is disclosed, for example, in Japanese Patent Publication, 39-3652 according to which spongy metallic bodies of a relatively high porosity could be obtained. In this method, a steel mold is filled with a powder of sodium chloride to form a powder compact and a melt of a metal having a relatively low melting point, such as aluminum, zinc and lead, is poured over the powder compact preheated at a suitable temperature under pressurization so as to penetrate into the interstices of the sodium chloride particles followed by cooling to effect solidification of the molten metal and to form a composite body of the metal as the matrix and the sodium chloride particles as the dispersant which is then leached away in water to leave the metallic matrix in a spongy form.
In this method, however, the metal should have a melting point lower than that of sodium chloride of 801.degree. C. and the shape of the product is also under limitation because the method necessarily involves the step of compacting of the sodium chloride powder under pressure so that the method is not suitable for the manufacture of spongy metallic bodies of complicated forms or large sizes. According to the teaching of the above recited Japanese patent, in addition, the preheating temperature of the sodium chloride compact should always be higher than the solidification temperature of the molten metal pressurized thereinto. As is described below, a very fatal defect is caused therefrom in practicing the pressurization of the molten metal into the powder compact of sodium chloride. This is presumably the principal reason for the fact that even a period of two decades or longer is not long enough for the practical development of the patented method in the industrial production of spongy metallic bodies.
When the pressurized casting method is applied to the preparation of a composite body composed of a metal matrix and a powder compact as a precursor of a spongy metallic body, it is a prerequisite to prevent premature solidification of the molten metal in the narrow interstices of the powder particles to clog the passage of the melt in order that the melt is able to flow threading the interstices and to reach the core portion of the body. It is therefore reasonable in this regard that the above mentioned Japanese patent teaches a preheating temperature higher than the solidification temperature of the metal melt. Such a preheating condition, however, can afford no solution of the problems given below. Namely, the melt under a pressure of at least several tens of kg/cm.sup.2 is readily pressurized into the narrow clearance between the metal mold and the plunger or the small air escape in the mold and violently ejected therethrough. When the plunger is further thrusted into the mold under an increasing pressure, the plunger eventually comes into contact with and pressing of the powder compact to cause destruction or plastic deformation thereof. Such undesirable phenomena can be avoided only by interrupting the pressurization immediately when leak of the melt takes place but interruption of pressurization naturally results in the loss of the desired effect of solidification under pressurization so that the thus obtained composite bodies are always not free from a number of portions unfilled with the metal and satisfactory products can hardly be obtained. Furthermore, intentional or accelerated cooling of the mold from outside is essential in order to effect efficient solidification of the molten metal as a result of the high preheating temperature so that the productivity of the process is not always sufficiently high.
The above described problems can be solved altogether when the preheating temperature of the powder compact is lower than the solidification temperature of the molten metal so as to utilize the powder compact per se and the mold as a heat sink, i.e. escape for the heat evolved by the solidification of the melt, although, needless to say, the preheating temperature cannot be too low in order to avoid the premature solidification of the melt to clog the interstices for the passage of the melt resulting in incomplete infiltration of the melt through the powder compact as is mentioned above.